1. Field of the Invention
The present invention relates to a method of breaking foam generated from polyvinylchloride (hereinafter referred to as PVC) paste resin latex. In further detail, the present invention relates to a method of breaking foams which is produced during the vacuum stripping process of residual monomers from foamable PVC paste resin latex after polymerization.
2. Prior Art
PVC paste resin latex has been used for making plastisol by dispersing the PVC paste resin in a plasticizer and heat gelating the plastisol to fabricate goods such as wall paper, floor coverings and gloves. PVC paste resin latex is manufactured by several methods such as a micro-suspension polymerization process, an emulsion polymerization process, or a seeds-emulsion polymerization process in which vinyl chloride monomers or a mixture of vinyl chloride monomers and comonomers polymerizable therewith are polymerized usually using an anionic or cationic surface active agent. The PVC paste resin latex comprises polymers having a particle size of 0.1 to 10 .mu.m and is subjected to spray drying. However, the PVC paste resin latex contains a great deal of residual monomer. Therefore, it is necessary to remove such residual monomer from the latex for the sake of recycling. Moreover, it is necessary to lower the monomer contents in the latex before drying for the sake of reducing vinyl chloride monomers being emitted to the environment.
Generally, as a method of removing residual monomers from PVC resin aqueous liquid, there is disclosed a method of applying steam to the slurry after polymerization under vacuum in Published Examined Japanese Patent Application No. Toku-Kou-Sho 53(1978)-38187 and a method of passing the slurry through perforated trays to contact with a countercurrent flow of steam in a multi-column stripping tower, the steam being introduced to the bottom of the tower, in Published Unexamined Japanese Patent Application No. Toku-Kai-Sho 54(1979)-8693.
There is also a method of recovering residual monomers batch-wisely under vacuum pressure by transferring the PVC resin aqueous liquid into a pressure vessel like a tank and raising the temperature to the extent that the slurry can maintain its stability.
These methods of steam blowing therein or vacuum recovery by heating have problems in that the PVC paste resin latex produces foam, and the foam flies into the vacuum pump line, resulting in the failure of or troubles with the equipment when the latex contains a great deal of a surface-active agent, which makes stable and continuous operation difficult.
Therefore, there existed a defect in that the time for recovery of the residual monomer increased because the treated quantity was reduced and the gas evacuation velocity was lowered in order to control such foaming. The PVC resin aqueous liquids of the present invention means the slurry or latex after polymerizing vinyl chloride monomers by a method of suspension polymerization, micro-suspension polymerization or emulsion polymerization.
As a method of breaking foam from the above-mentioned foamed PVC paste resin latex, there have been proposals of a method of adding proper antifoaming agents and a method of mechanical breaking of foam. However, it has been impossible to control foaming by the method of adding anti-foaming agents to the PVC paste resin latex, since such anti-foaming agents can only be used in a small amount because due to the problem of deteriorating product quality such as heat-stability. Moreover, the method of mechanical breaking of foam sometimes causes degradation of product quality due to breakage of the latex by collision and shearing.
There is proposed another effective method of breaking foam as disclosed in Japanese Patent Application No. Toku-Gan-Hei 11(1999)-12656, wherein foam generated from the latex is broken by passing the latex through the space of the vacuum recovery device or cooling portion in an evacuation pipe. However, in the case that a steam blowing method is employed as the vacuum recovery method, there is a problem that the steam is condensed at a cooling portion resulting in the reduction of the solid concentration of the latex and in low productivity of the spray drying machine in the after-treatment process.